Fast-charging, battery charging systems are distinguished from other battery charging systems in that they operate to produce a battery charging output with a higher kilowatt output and approximately twice, or greater, the charging rate than traditional battery charging systems. An industrial-type, fast charging, battery charging system can include a power supply connected to one or more charging stations, and the charging stations can have output currents up to 500 A or greater, and power outputs up to 30 kW and greater. Compatible battery voltages are typically 12 to 80 volts from a lead-acid battery or battery bank. The industrial-type, fast charging, battery chargers can typically be used for charging lift trucks, fork lifts, golf carts, and the like, which chargers operate at relatively higher electrical power levels to charge a 12-80 volts direct current (VDC) battery system. In these systems, the battery is the main power source for driving the fork lift, golf cart, and the like.
These fast charging systems can have a primary side switched-mode power supply that converts a mains alternating current (AC) electrical power into a suitable direct current (DC) electrical power. In general terms, the fast-charging, battery charger, power supply can include input terminals for mains input, and an input rectifier and filter for filtering and rectifying the mains input, an inverter for converting the rectified input power to a higher frequency, a high frequency transformer which converts the voltage up or down to the required output level on its secondary winding(s), and another rectifier and/or filter to provide a suitable DC battery charging power. Mains power can be 120, 240, 480, 600, or higher, VAC, and single phase or multiphase being typical for the higher voltages. A switched-mode power supply has the advantage of providing a relatively high frequency to the transformer, which allows the transformer to be smaller for a given current capacity, as transformer size is inversely related to operating frequency.
Consequently, high frequency transformers operating at high voltages and high currents are commonly used in battery-charger power supplies. The output stage of a battery-charger power supply, for example, may include an electrical transformer to transform the high bus voltage of the battery-charger power supply into a high current charging output. Transformer primary coil voltages on the order of 465 volts at 20 to 100 Khz and secondary coil currents on the order of 400 amps can be typical, but other voltages and frequencies are possible. As such, battery charger power supply transformer coils (e.g., primary and secondary coils) are made from large diameter wires (3-14 gauge wire is typical) in order to handle the currents generated by these large voltages.
Most of these transformers include a central bobbin having a coil winding window disposed about a central opening in the bobbin. The central opening is provided to receive one or more laminated or ferrite magnetic cores. Standard off-the-shelf magnetic cores are available in a wide variety of sizes and shapes, many of which have square or rectangular cross-sections. The coil windings typically also have rectangular or square cross sections wound close to the magnetic cores. This is because it is generally desirable to keep the coil windings close to the magnetic core to maximize the magnetic coupling between the magnetic core and the coil windings.
Having coil windings with rectangular or square cross sections can be problematic in charging applications however. This is because the large diameter wires used in battery-charger power supply transformers have a tendency to deform or bulge at locations where the winding direction changes quickly (e.g., at the corners when wound around a bobbin having a square or rectangular cross section). This is especially true for Litz wire, a stranded woven type of wire used extensively in high frequency (e.g., 20 Khz to 100 khz) battery charger power supply transformers. The outer insulation that is placed over these large wires can also bulge and deform.
The width of the overall coil winding in the area of the deformations tends to be wider than the width of the remaining portion of the coil because of the bulging wires. As a result, the coil may not fit within the winding window of the bobbin in those areas. At the very least, extra manufacturing steps, typically manual, must be taken during the coil winding process to properly fit the deformed coil into the winding window in the vicinity of the bulges or deformations. It is desirable, therefore, to have a bobbin winding window cross section that does not have quick changes in winding direction. Preferably, the central opening in the bobbin will still accommodate standard size, readily available, magnetic cores having rectangular or square cross sections.
Another problem with using large diameter wires in battery-charger power supply transformers is that the wire leads to and from these transformers tend to be less flexible than smaller wire leads. Extra space has typically been available inside of the battery-charger, power supply chassis around these transformers to allow the high-voltage and high-current transformer leads to be safely routed and connected to the rest of the battery-charger power supply.
The current trend in designing battery-charger power supplies, however, is to make these devices smaller. One way to accomplish this is to pack the various power supply components closer together inside of the chassis. As a result, other power supply components are placed closer to the high-voltage, high-current transformers in these designs. Thus, less room is provided to safely route the leads from the transformer to the rest of the power supply.
It is desirable, therefore, to have a battery-charger, power supply transformer wherein the leads exit the transformer in a known and repeatable manner. Preferably, the transformer structures will have smooth edges and surfaces in the vicinity where the leads exit the transformer to prevent damage to the transformer leads.
Another problem with battery-charger, power supply transformers, especially battery-charger, power supply transformers operating at high frequencies, is leakage inductance. The presence of high leakage inductance in these transformers can cause several problems. A leaky output transformer can reduce the output power of the battery charger power supply. The primary and secondary coils in leaky transformers are more susceptible to overheating. Finally, the energy stored in the leakage inductance can be detrimental to transistor switching circuits in the battery-charger power supply. Release of this stored energy can cause ringing, transistor failure and timing issues. Reducing or minimizing the leakage inductance in battery-charger, power supply, transformers is therefore generally desirable.
Leakage inductance results from primary coil flux that does not link to the secondary coil. The amount of primary coil flux linked to the secondary coil is dependent on the physical orientation and location of the primary and secondary coils with respect to each other. Reducing or minimizing the mean distance between the turns of the primary coil and the turns of the secondary coil will typically reduce or minimize leakage inductance in a transformer. Reducing or minimizing the mean length of the turns in a coil will also typically reduce or minimize leakage inductance.
It is desirable, therefore, to reduce or minimize the mean distance between the turns of the primary coil and the turns of the secondary coil in battery charger power supply transformers. Preferably, the mean length of the turns in the coils of the transformer will also be reduced or minimized.